1. Field of the Invention
The present invention relates to the field of ink transfer printing and, more particularly, to color ink transfer printing which is driven by a viscosity change in ink.
2. Description of the Related Art
Over the years, many attempts have been made to develop a printing technique with a simple mechanical structure, the hope being that such a printing technique would lead to reliable, low cost products. For one reason or another, known printing techniques have not been able to fully satisfy these goals.
Known thermal printing techniques have certain drawbacks. Direct thermal printing, for example, requires special heat sensitive paper. Thermal transfer printing inefficiently uses ink ribbons, which leads to higher cost for ribbon usage, especially for color printing. Another disadvantage of thermal printers is that their printing speed is too slow for large volume printing.
Ink jet printing has many advantages, but continues to have some reliability concerns. A major disadvantage of ink jets, namely bubble-type ink jet devices, is that deposits form in the nozzles when the organic compounds in the ink break down at high temperatures (e.g., 350.degree. C.). The high temperatures are needed to produce the bubbles which cause a drop to be ejected. As a result, the bubble-type ink jets tend to have a clogging or crusting problem at the nozzles. Another disadvantage of ink jets is that the ink used must have a low viscosity (e.g., typically &lt;10 centipoise) which severely limits the type and variety of inks which may be used. The printing speed of ink jets is also too slow for large volume printing.
Additional background information on known printing techniques may be found in "Computer Graphics Technology and Applications," Vol. II--"Output Hardcopy Devices," by Robert C. Durbeck and Sol Sherr, San Diego 1988.
In Hori, U.S. Pat. No. 4,608,577, an ink jet type thermal printing machine is described. The printing machine includes a film or belt having a plurality of holes which correspond to the conventional ink jet nozzle. The holes in the film or belt are filled with ink. The ink in the holes is then heated (vaporized) until bubble pressure causes the ink to be jetted out onto paper.
In Bupara, U.S. Pat. No. 4,675,694, a printer is described. The printer includes a perforated printing plate, individual heaters, and an ink container. The ink utilized is a phase-change or hot-melt ink which is a solid at room temperature. The printer operates on the principle that when a portion of the solid ink contained in the holes of the printing plate is heated, it undergoes a volume expansion (due to its change from a solid state to a liquid state) which causes ink to protrude out of the holes which have been heated. After the ink has been expanded in certain holes, a printing medium is brought into contact with the liquified ink for transfer thereto. The printing medium must be brought into contact with the liquified ink before the ink is allowed to cool. Alternatively, the printing medium may be placed in contact with the printing plate prior to the volume expansion.
In Cielo et al., U.S. Pat. No. 4,275,290, a thermally activated liquid ink printer is described. The printer includes an ink reservoir having a plurality of orifices. The driving force of the printer is the application of localized heat to ink in an orifice which causes at least partial vaporization of the ink and/or reduction in the surface tension. As a result, ink flows out of the orifices being heated. Preferably, the heating produces bubbles which cause ink drops to be ejected. Alternatively, the heating acts only to reduce surface tension which causes ink to flow through the orifices being heated. This alternative operation uses a hydrostatic pressure which is less than the surface tension of unheated ink at the ink surface.
In Cielo et al., U.S. Pat. No. 4,164,745, a method is described for varying the amount of ink deposited on a sheet of paper moving past an orifice based on the viscosity of the ink. For example, the width of a line being printed can be controlled. The method described is unable to completely control the flow of ink. That is, the flow of ink is continuous. It is only the amount of ink flowing which is variable. In this regard, Cielo et al. ('745) provides a bypass to prevent the continuous flow of ink from flowing out of an orifice across a gap onto paper, but only while the viscosity is above a predetermined value. The ink which does flow out from an orifice must cross a gap before it reaches the paper.
As will become more apparent below, the present invention provides a novel and nonobvious technique for printing which has the potential for a wide range of applicability. The present invention overcomes many of the disadvantages of known printing techniques because it has not only a simple and reliable design, but also the ability to use a wide variety of inks to achieve high resolution printing. The technology associated with the present invention is applicable to printers, digital copiers, video printers, facsimile machines, and the like. Color and gray scale printing or copying are also available with this technology.
Furthermore, the known prior art fails to appreciate the advantages and benefits of viscosity driven printing. The present invention can easily and accurately control printing by altering the viscosity of the ink. Such control is superior to that provided by bubble, phase-change or surface tension driven printing. Cielo et al.('745) assumes that the ink always flows through the orifices and attempts to control the amount of ink deposited on paper using ink viscosity. Bupara operates on volume expansion solid ink as its phase changes from solid to liquid. The Bupara technique is very cumbersome and time consuming in that it requires a large number of procedures to transfer ink to paper. Cielo et al.('290) operates either in a vaporization mode in which ink is ejected out of orifices across a gap to paper or in a surface tension and pressure mode in which ink under pressure flows out of orifices when heat is applied to the orifices. The vaporization mode of Cielo et al. is similar to a bubble-type ink jet and, therefore, very dissimilar to the present invention. The surface tension and pressure mode of Cielo et al. has questionable operability. Namely, it is unclear how ink transfer will take place because the paper never contacts the orifice plate. Furthermore, it would be very difficult, if not impossible, to construct a practical printer which relies on surface tension to control the printing process.
Although it is known that surface tension and viscosity vary with temperature, the magnitude of change between surface tension and viscosity is drastically different. Table 1 (below) shows that the viscosity change of fluids resembling ink is quite drastic over an 80.degree. C. temperature change in comparison to the slight change in surface tension. The well known fluids of glycerol and ethylene glycol are used to model characteristics of inks which would be useful in the present invention.
TABLE 1 ______________________________________ VISCOSITY SURFACE TENSION (cps) (dyne/cm) ______________________________________ Temperature 20 100 20 100 (Celsius) Glycerol 1410 16 63.3 .about. 60 Ethylene Glycol .about. 19 2.3 48.43 41.31 ______________________________________
As shown in Table 1, the magnitude of change in viscosity far exceeds the change in surface tension. This large magnitude of viscosity change provides a high level of control which is necessary to manufacture a reliable, low cost product. In contrast, since the magnitude of change in surface tension over a reasonable temperature change is not significant from a design or engineering standpoint, a device relying on surface tension would be difficult to control.
In sum, the known prior art fails to appreciate the benefits and advantages of a printer which is driven by a viscosity change in ink.